Numerous applications require high brightness, high frequency electron sources. Those include military, aerospace, communications and other commercial industries. Advances in modern technology require higher performance of electron sources that can be used for generation of powerful microwave radiation. One of the ways to achieve a high-brightness electron beam with desired parameters is to use a photocathode irradiation technique. However, this method produces low electron currents (emittance), much lower than that of thermionic cathodes, which limits a range of possible applications.
U.S. Pat. No. 4,313,072 describes an electron gun in which the electron beam is modulated by laser pulses illuminating a photocathode. Electrons are generated by the photocathode, and the electron current is limited by the performance and properties of the photocathode, resulting in current density that is usually low. U.S. Pat. Publication No. US2002/0053867 A1 discloses a separate cathode for emitting electrons and an electron beam guidance cavity for concentrating electrons, which uses an insulating material around the cavity exit aperture such that the insulating material is a coating (e.g., MgO) having certain secondary electron emitting properties. The output current density J of such an electron source depends on the diameter (area) of the exit aperture, thus making it possible to obtain high values of J with small apertures. However, it is difficult to achieve high frequency modulation of the beam using this approach. The thermal spread of electron energies will limit the cut-off frequency in case of thermionic cathodes. The problem with using cold cathodes in this application is the cathode-to-grid capacitance, which leads to a low input impedance at higher frequencies.